The invention relates generally to apparatus and methods for imaging in the Terahertz (THz) frequency range, and more specifically to examining a sample containing an explosive material.
Over the past several years, there has been an emerging interest in the potential of THz detection for security related applications such as imaging of concealed weapons, explosives and chemical and biological weapons. Terahertz radiation is readily transmitted through most non-metallic and non-polar media, which advantageously enables the THz systems to “see through” concealing barriers such as packaging, clothing, shoes, book bags, for example, in order to probe any potentially dangerous materials contained within. Additionally, many materials of interest for security applications including explosives and chemical and biological agents have characteristic THz spectra that may be used to fingerprint and thereby identify these concealed materials. Thus, the combination of transparency to clothing and packaging combined with spectroscopy of illicit materials such as narcotics, biological weapons or explosives may facilitate detection and identification of many different types of materials. Furthermore, THz radiation is believed to pose no more than minimal health risks to either a person being scanned or the operator of the system.
Presently available THz imaging techniques employ receivers that can be operated in a super-heterodyne configuration. The super-heterodyne approach is a relatively simple and low-cost approach that uses a single low noise amplifier (LNA) gain stage, a mixer, a local oscillator (LO) source and an intermediate frequency (IF) block. However, due to the relatively large number of receiver channels required, time synchronization of the local oscillation distribution may be critical. Direct-detection architectures may be employed to overcome the shortcomings of the super-heterodyne receivers. The direct-detection architecture entails use of a high gain LNA cascade, bandpass filtering, a high sensitivity detector and direct current (DC) electronics for noise voltage amplification. Unfortunately, these direct-detection receivers require a high level of gain in the low noise amplification stages with a flat gain response in order to amplify the scene noise floor above the noise floor of a detector diode. Furthermore, controlling this level of gain at the module level may be an onerous task as oscillation and unwanted feedback may occur especially as the operating frequency is increased.
There is therefore a need for a THz imaging system capable of real-time imaging. In particular, there is a significant need for a design of a THz imaging system for real-time imaging for use in security standoff detection applications. Also, it would be desirable to develop a simple and cost-effective method of fabricating a THz imaging system capable of real-time three-dimensional imaging.